For many years research has been conducted with view to developing new energy production systems that counter the steady depletion of oil reserves and that at the same time take into account current and future requirements in combating pollution and the associated climate changes.
In this regard, fuel cells constitute a non-polluting source of electricity. They are current generators of the electrochemical type, the operating principle of which consists in passing a fuel, generally hydrogen, over an anode so as to generate, in the presence of a catalyst (such as platinum), protons which, after they have passed through an electrolyte such as a polymer membrane come into contact with an oxidizer, generally oxygen, which is generated at the cathode, in order to produce water, in combination with electrons which also come from the anode. The water is discharged from the system, as is the heat generated. The circulation of the electrons from the anode to the cathode produces electricity.
The hydrogen used in fuel cells constitutes an almost inexhaustible planet-wide element. It can be produced by the electrolysis of water by renewable energy means, for example hydroelectric, wind or solar power, or by means of a source of nuclear power. Alternatively, it can be produced by the reforming of natural gas or by the gasification and then thermochemical reforming of biomass.
Fuel cells may find various applications, in particular in the development of “clean” vehicles, that is to say those not emitting carbon dioxide or other pollutants potentially harmful to the environment or to human health, or in the development of low-power portable systems such as mobile telephones and portable computers.
In general, a fuel cell comprises a stack of several cell elements, each of which consist of an electrolyte, such as a proton exchange membrane or PEE, sandwiched between the anode and the cathode, the whole assembly constituting an active core or MEA (Membrane/Electrode Assembly) which is itself contained between two conducting plates. Current collectors are provided at the ends of the stack of cell elements.
The conducting plates include channels into which the feed gases flow and from which the water is discharged. They may be of the monopolar or bipolar type. The term “monopolar” is understood to mean that each cell element comprises an anode plate and a cathode plate in direct contact (or in contact via a separator consisting of a corrugated conducting metal plate) with the respective cathode and anode plates of the adjacent cell elements. The term “bipolar” is understood to mean that each plate comprises an anode face and a cathode face and, by itself, ensures connection between two adjacent cell elements. In general, a coolant is made to circulate in channels formed in the bipolar plate or formed between two adjacent monopolar plates or else in passages provided in the corrugated separator plate.
A gas diffusion layer or GDL, consisting of conducting fibres, is in principle interposed between each electrode and the plate that faces it.
The conducting plates are conventionally made of graphite or a carbon/polymer composite, or metal plates.
Graphite makes it possible to achieve good performance because of its high electrical and thermal conductivities and its low contact resistance. It can also easily be machined, offering the possibility of optimizing the geometry of the channels passing through it. Finally, its high corrosion resistance guarantees the longevity of the cell element core. However, machining graphite is a lengthy and expensive operation, whatever the volume of production, its use being reserved for laboratory cell elements and a few prototypes.
A carbon/polymer composite allows plates to be obtained by moulding, which considerably reduces the costs. However, although 10 to 20 times less expensive than graphite plates of the same dimensions, carbon/polymer composite plates remain too heavy for mass markets, such as the automobile market. Moreover, the performance of this material is inferior to that of graphite, because of a lower conductivity and less favourable channel geometries as result of the manufacturing constraints.
Stainless steel in fact constitutes the material the best suited to the mass production of fuel cells, with a cost from 5 to 10 times less than that of carbon/polymer composites. It allows the production of plates offering good electrical and thermal conduction, combined with satisfactory mechanical strength of the cell elements. It also has the advantage of being lightweight, occupying little space and able to be subjected to many proven assembly techniques.
However, stainless steel has certain drawbacks that remain to be overcome, namely:                the electrical surface resistance of this material, inherent due to the presence of an oxide layer (or passive film) on the surface of the metal, reduces the performance of the cell elements as a result of ohmic losses occurring at the interface between the conducting plate and the gas diffusion layer;        corrosion of the metals, in particular chromium, in the acid reducing medium of the fuel cell releases cations which, by contaminating the cell membranes, limit their lifetime; and        limited optimization of the plate geometry because of difficulty in forming the material.        
It has already been suggested in Application U.S. 2002/0172849 to use nickel-chromium austenitic alloys to manufacture bipolar plates having high conductivity and good corrosion resistance. These alloys contain at least 50% by weight of chromium and nickel, the nickel being preferably predominant. The best properties of these alloys, compared with 316 stainless steel conventionally recommended for this use, are attributed to their high nickel content.
Moreover, a comparison of the performance of bipolar plates having various chemical compositions has been presented in the publication by D. P. Davies et al., “Stainless steel as a bipolar plate material for solid polymer fuel cells”, Journal of Power Sources 86 (2000) 237-242. Among the 316, 310 and 904L stainless steel plates tested, containing 18 to 25% chromium and 12 to 25% nickel, that made of 904L stainless steel proved to deliver a power density close to that of graphite plates and markedly superior to that of 316 stainless steel. This better performance of 904L steel is attributed to the higher proportion of nickel and chromium that it contains, its performance being assumed to result from a thinner oxide film, which therefore has a lower resistivity.
Furthermore, Application FR-2 860 104 has suggested a fuel cell configuration making it possible to obtain a high surface area for contact between a monopolar plate and the corresponding diffusion layer, and thus to reduce the electronic contact resistance between these two elements with a view to improving both the efficiency and the power of the cell for a given current density. To do this, the surface of the plate is roughened by sandblasting, filing or abrasion, so as to be suited, or even matched, to that of the diffusion layer. This post-treatment is potentially more expensive than the “native” treatment according to the invention.
However, there remains the need to have stainless steel conducting plates that allow the manufacture of fuel cell elements having a performance and a lifetime that are comparable to those of cell elements comprising machined graphite plates, whilst still being less expensive.